How can I use Photoshop to edit the textures of my model?
You can sort of "open" 3d model in Photoshop (example), but it's not something as flexible as editing it with 3d package (and that won't provide you with most features like what is expected when importing 3d models) . What Blender provides you with is to edit the texture of the model with any image editing program. There are 2 possible ways:


editing image used as texture directly in image editing software. Note that you might want to export UV layout in order to be able to see what faces you're painting on;
export current position of the mesh in the viewport to the image editing program, paint on the mesh like it was visible when captured in 3-rd party program and then apply those changes back to the image texture in Blender. This feature is called Quick Edit (image painting program should be set up in the User Preferences):